Anita Wells shuffles cautiously up to the edge of the glass floor and then stops short. The view before her of the Grand Canyon thousands of feet below causes her to tremble. "Oh, I can't do this," she moans.

But Ms. Wells and her sons have arrived at 6:30 a.m. on this snow-flurried spring morning to be among the first tourists to step out onto the horseshoe-shaped bridge. And so, with her son Adam's prodding, Wells takes her first few steps gingerly. She feels a comforting sturdiness beneath her in the three-inch-thick glass. Then, minutes later, she's smiling and laughing at the far end of a structure cantilevering off the West Rim of the world's most famous chasm.

"I was really nervous about doing this, but my boys wanted to so I figured I should try something new," says Wells, who is on a road trip from Atlanta with her 21-year-old twins. "Once I got out there, I got used to it, and then it was kind of a charge to be doing this."

Almost overnight, the glass-and-steel oxbow protruding out over the lip of the Grand Canyon has become one of the world's most unusual curiosities. Part high-wire act and part window into the womb of the Earth, the structure represents a new and controversial twist on the budding eco-tourism movement.

The Hualapai Indians, who consented to allow investors to build the $30 million Skywalk on their land, hope it draws thousands of visitors a year and brings a lift to their isolated reservation 120 miles southeast of Las Vegas. They're counting on it to create jobs and provide much-needed revenue for the 2,000 tribal members spread across 1 million acres of Arizona.

But critics, including some tribal members, consider it an affront to one of the world's most hallowed pieces of earth. "I'm not trying to denigrate their need, but this is designed to provide a thrill of being able to walk over the edge," says Robert Arnberger, a retired superintendent of Grand Canyon National Park. "I dislike the motivation behind it."